1. Field of the Invention
The present invention relates to a magneto-optical reproducing apparatus for reproducing information recorded on a magneto-optical medium utilizing a magneto-optical effect, and particularly to a magneto-optical reproducing apparatus for reproducing the information using a reproducing method by domain wall replacement.
2. Related Background Art
As a rewritable high-density recording medium, attention has heretofore been paid to a magneto-optical medium in which magnetic domains are formed in a magnetic thin layer thereof using thermal energy of a semiconductor laser to record information, and this information is read out by utilizing a magneto-optical effect. In recent years, there has been a strong demand for further enhancing the recording density of this magneto-optical medium to provide it as a recording medium having a greater capacity. The linear recording density of an optical disk such as the magneto-optical medium greatly depends on the laser wavelength and the numerical aperture of an objective lens of an optical system for reproduction. More specifically, since the laser wavelength xcex and the numerical aperture NA of the objective lens of the optical system for reproduction decide the diameter of a beam waist, the detectable range of the spatial frequency upon reproduction of signals is limited to about 2 NA/xcex.
Therefore, for actually achieving higher recording density with a conventional recording medium, it is necessary to shorten the laser wavelength xcex or enlarge the numerical aperture NA of the objective lens in the optical system for reproduction. However, the improvements in the laser wavelength xcex and the numerical aperture NA of the objective lens are limited naturally. Therefore, techniques that the structure and reading method of a recording medium are devises to improve the recording density have been developed. For example, Japanese Patent Application Laid-Open No. 6-290496 proposes a method for reproducing high-density record signals by displacing a domain wall present at a boundary between record marks utilizing a temperature gradient, and detecting this domain wall displacement. According to this method, the domain wall is displaced just at the time the domain wall has been heated to a temperature at which the domain wall is displaced, and the high-density record signals are reproduced by detecting this domain wall displacement, so that the signals can be reproduced without depending on the optical diffraction limit at all. In addition, since no waveform interference occurred even when high-density record signals are reproduced, complicated processings such as waveform equalization is unnecessary, and so the reproduction of information can be performed by a simple signal processing circuit.
On the other hand, when a track pitch is narrowed in order to enhancing a track density in the conventional recording media, crosstalk from adjacent tracks is overlapped on reproduction signals. Even the use of the above-described reproducing method described in Japanese Patent Application Laid-Open No. 6-290496 has involved a problem that crosstalk is also overlapped on reproduction signals when a track pitch is narrowed, and so information cannot be exactly reproduced.
It is an object of the present invention to provide a magneto-optical reproducing apparatus by which the above-described problems involved in the prior art, and higher-density information can be reproduced by not only enhancing a linear recording density, but also enhancing a track density.
The above object can be achieved by the present invention described below.
According to the present invention, there is thus provided a magneto-optical reproducing apparatus for reproducing information recorded on a magneto-optical medium comprising a transparent substrate, a first magnetic layer, a second magnetic layer and a third magnetic layer which layers are laminated on the substrate in that order, the first magnetic layer comprising a magnetic film relatively smaller in domain wall coercivity and greater in domain wall displaceability at ambient temperature and temperature close to ambient temperature compared with the third magnetic layer, and the second magnetic layer comprising a magnetic film having a Curie temperature lower than the first magnetic layer and the third magnetic layer, the apparatus comprising:
an optical system for irradiating the medium with a light beam, domain walls of a magnetic domain for information formed on the medium being displaced by the irradiation with the light beam;
a photosensor for detecting a change in a polarization direction of reflected light of the light beam to output a reproduction signal;
a differentiating circuit for differentiating the reproduction signal outputted from the photosensor; and
a binarizing circuit for generating a binary signal by comparing the signal differentiated in the differentiating circuit with a prescribed threshold value.
According to the present invention, there is also provided a magneto-optical reproducing apparatus for reproducing information recorded on a magneto-optical medium comprising a transparent substrate, a first magnetic layer, a second magnetic layer and a third magnetic layer which layers are laminated on the substrate in that order, the first magnetic layer comprising a magnetic film relatively smaller in domain wall coercivity and greater in domain wall displaceability at ambient temperature and temperature close to ambient temperature compared with the third magnetic layer, and the second magnetic layer comprising a magnetic film having a Curie temperature lower than the first magnetic layer and the third magnetic layer, the apparatus comprising:
an optical system for irradiating the medium with a light beam, domain walls of a magnetic domain for information formed on the medium being displaced by the irradiation with the light beam;
a photosensor for detecting a change in a polarization direction of reflected light of the light beam to output a reproduction signal;
a first differentiating circuit for differentiating the reproduction signal outputted from the photosensor to generate a primary differential signal;
a second differentiating circuit for differentiating the primary differential signal to generate a secondary differential signal; and
a binarizing circuit for generating a binary signal by comparing the secondary differential signal generated in the second differentiating circuit with a prescribed threshold value.